The present invention is directed, in general, to geophysical exploration and, more specifically, to a system and method for orienting seismic energy sources and receivers to yield discriminated compressional and vertical shear waves.
Currently, most geophysical techniques dealing with multi-dimensional seismic data do not discriminate or locate seismic energies of different orientations, such as the compressional energy or vertical and horizontal shear energies of reflected seismic data systems. In a typical multi-dimensional seismic survey, a multi-mode seismic energy generator may be used to generate a preponderance of one orientation of seismic energy relative to a particular orientation. Then a preponderance of energies orthogonal to the first but relative to the same orientation may also be generated. However, the orientation of the received seismic energies changes at each receiver due to the fact that the orientation between the seismic energy source and each receiver in a multi-dimensional seismic array is different.
Differently oriented seismic energies travel differently through the subsurface strata based upon the characteristics of the subsurface strata. Thus, if a fault plane in the strata were in more of a vertical orientation relative to the plane defined by the seismic energy source-receiver line, the different seismic energies would have a different travel characteristic. The mapping of the subsurface features could be greatly enhanced through processing of the oriented seismic energies mentioned above. This is especially true in an orientation specific to the azimuths defined by each seismic energy receiver and source pair. Additionally, important rock property information can be ascertained by comparing differences and similarities of the attributes of the oriented seismic energies.
Compressional energy waves may generate vertical shear energy waves at subsurface interfaces. Additionally, vertical shear waves may split into two xe2x80x9cdaughterxe2x80x9d shear waves in areas containing subsurface vertical fractures that complicate the problem of intermingling but offer opportunity for analysis if the energies could be discriminated. However, the processing of such data is complicated, because the oriented energies are intermingled and therefore not easily discriminated into the differently oriented energies for each receiver-source azimuth. Also, the processing of these components is further complicated since the orientation of the operational modes of the seismic energy source do not always correspond to the orientation of each and every receiver in the geophysical array.
Accordingly, what is needed in the art is a way to more effectively separate and discriminate between compressional and vertical shear energy orientations in seismic surveying situations.
To address the above-discussed deficiencies of the prior art, the present invention provides a system for, and method of, orienting seismic energy receivers to substantially separate a compressional wave from a vertical shear wave. In an advantageous embodiment, the method includes reflecting a seismic energy from a subsurface interface to produce a reflected seismic energy wave, such as one generated by an energy source, that has a compressional energy and vertical shear energy associated therewith. A first seismic energy receiver is oriented such that it is aligned with an angle of emergence of the reflected seismic energy wave to thereby maximize the vertical shear energy received by a second seismic energy receiver. The emergence angle may vary, for example, between 0xc2x0 and 90xc2x0. As discussed below, the orientation of the first receiver may be either physical or its orientation may also be achieved by applying an algorithm to the data received by the first seismic energy receiver. In a preferred embodiment, the first and second seismic energy receiver are orthogonal with respect to each other.
In another embodiment, orienting the first and second seismic energy receivers includes orienting the first and second seismic energy receivers in a vertical plane containing the energy source and said first and second seismic energy receivers.
In another embodiment, the method includes applying an algorithm to data received by the first and second seismic energy receivers. In such embodiments, the algorithm may be used to convert data received from a radial coordinate to a compressional coordinate of the first seismic energy receiver and convert data received from a vertical coordinate to a vertical shear coordinate of the second seismic energy receiver. In an alternative embodiment, however, the step of orienting the first and second seismic energy receivers may include physically orienting the first seismic energy receiver with the emergence angle. It should be understood that orienting the first seismic energy receiver with the emergence angle will also change the orientation of the second seismic energy receiver with respect to the emergence angle.
In another aspect, the present invention also provides a system for separating a compressional seismic wave from a vertical shear seismic wave. In an exemplary embodiment, the system includes a reflected seismic energy wave reflected from a subsurface interface and that has compressional energy and vertical shear energy associated therewith and further includes first and second seismic energy receivers wherein the first seismic energy receiver is aligned with an angle of emergence of the reflected seismic energy wave to thereby maximize the vertical shear energy received by the second seismic energy receiver. In yet another aspect, the present invention provides a method of exploring a subterranean feature with seismic energy. In this embodiment, the method includes (1) generating a seismic energy wave toward a subterranean feature (2) reflecting the seismic energy from the subterranean feature to produce a reflected seismic energy wave having compressional energy and vertical shear energy associated therewith and (3) separating at least a portion of a compressional wave of the seismic energy wave from at least a portion of a vertical shear wave of the seismic energy wave, which further includes orienting first and second seismic energy receivers such that the first seismic energy receiver is aligned with an angle of emergence of the reflected seismic energy wave to thereby maximize the vertical shear energy received by the second seismic energy receiver.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.